The field of allogeneic hematopoietic cell transplantation (HCT) has dramatically evolved over the last decade. The successful development and application of non-myeloablative conditioning regimens has permitted a broader range of patients to undergo this form of cellular therapy. The isolation and characterization of progenitor populations and donor cells that confer beneficial anti-tumor effects without graft-versus-host disease (GVHD) promises to move the field towards the use of engineered grafts, and major inroads have been made in the use of alternative donors who are not completely matched at the MHC, expanding the possibility that haplo-identical and cord blood transplantation will become routine in the next 10 years. Along with these developments are new challenges. One major obstacle to the forward progress of HCT in all of these areas is the greater engraftment resistance encountered when the intensity of the regimen is reduced and/or the grafts are modified or transplanted between donor and recipients of increased genetic disparity. Our laboratory has studied in preclinical mouse models resistance to engraftment of purified allogeneic HSCs and ways to overcome the barriers to engraftment. Recent data show that we can now segregate the effects of immune mediated resistance from non-immune elements which prevent donor cell engraftment. The current grant aims has three Specific Aims: (1) To study the immune and non- immune barriers that resist engraftment of allogeneic HSC;(2) based upon our understanding of the mechanisms of resistance to develop novel non-myeloablative regimens;and (3) to characterize by phenotype and function the non-stem cell populations that facilitate allogeneic HSC engraftment. The overall goal is to utilize the experimental systems we have developed to further understand the biology of how hematopoietic cells are recognized and resisted. Based upon our understanding of the immune and non- immune barriers to engraftment we have a long-term goal of making allogeneic HCT safer and more effective for the treatment of malignant and non-malignant disease.